The percent of measurement error (missing heart rate) per day is found in the reports by the heart rate graph. In a measurement of good quality, there are less than 10 % of missing heart rate, of moderate quality 10-15 %. When there are more than 15 % of missing heart rate, the reliability starts to be compromised, and at the latest when the percent is > 20 % on all measurement days, Firstbeat recommends making a new measurement. If there is a long measurement break (several hours), it shows up as increased missing heart rate %, but does not otherwise affect the reliability of the results.
The most typical error comes from bad electrode contact (hair has not been removed from the electrode site or the skin is greasy or dirty, or the cable is too tight and causes constant “pull” on the electrodes); in these cases, there is usually constant error throughout the measurement or at least when the person is moving about. If there is little or no error during sleep, the reason is usually bad contact, which becomes more obvious when the person is moving. If there is steady error throughout the measurement day and night (50-70 % or more), a technical flaw with the device is suspected. A high percent of missing heart rate (e.g. 30-40 %) can also imply ectopic beats of the heart or other irregularities in heart function, which prevents reliable detection of the R-wave. This is suspected especially if the other possible causes have been ruled out and there is no indication of bad electrode contact.
If you suspect that the problem is with the device, you can test the device with a short measurement. If the device works normally during the test measurement and does not show high error percentage, the cause of the error is most likely some of the above mentioned. However, if the test measurement shows a lot of error, it might be good to send it to repair. You can fill in the report for faulty devices from here.